A disk drive, such as a magnetic disk drive, comprises a magnetic disk, spindle motor, and magnetic head. The magnetic disk for use as a recording medium is arranged in a case. The spindle motor supports and rotates the magnetic disk. The magnetic head reads data from and writes data to the magnetic disk. The magnetic head is supported by a pivotable head actuator and configured to be moved radially relative to the magnetic disk and positioned in place.
Magnetic heads for perpendicular magnetic recording and thermally-assisted magnetic recording systems have recently been proposed in order to increase the recording density and capacity of a magnetic disk drive or reduce its size. One such magnetic head comprises a near-field transducer, which emits near-field light toward a recording layer of the recording medium and a waveguide for propagating light for the emission of the near-field light. According to this magnetic head, the medium recording layer having a perpendicular magnetic anisotropy is locally heated by the near-field light emitted from the distal end of the near-field transducer during writing data. In this way, the coercive force of the recording layer portion is fully reduced, so that a high recording density can be achieved.
The distance (transducer-pole distance) between a heating spot center of the near-field transducer and a main pole end along the track of the magnetic disk changes according to the rotational position of the magnetic head. Thus, the skew angle changes according to the rotational position of the magnetic head, so that the transducer-pole distance varies depending on the position of the magnetic disk, whether inner peripheral, intermediate peripheral, or outer peripheral, where the magnetic head is located. The transducer-pole distances in the inner and outer peripheral positions are longer than that in the intermediate peripheral position.
Further, the moving speed of the magnetic recording head relative to the magnetic disk, that is, the peripheral speed of the disk, varies according to the radial position of the disk. The peripheral speeds in the inner and outer peripheral positions are lower and higher, respectively, than that in the intermediate peripheral position.
In thermally-assisted recording, there exists a time T ((transducer-pole distance)/peripheral speed) of medium travel between the heating spot center and a pole end surface that ensures an optimal signal-to-noise (S/N) ratio. As described before, however, the transducer-pole distance and peripheral speed vary depending on the position of the magnetic disk, whether inner peripheral, intermediate peripheral, or outer peripheral. Therefore, it is difficult to obtain an optimal medium travel time or optimal S/N ratio for every radial position. In the inner peripheral position, for example, the transducer-pole distance and peripheral speed are long and low, respectively, so that the medium travel time T ((transducer-pole distance)/peripheral speed) is longer than in the intermediate peripheral position, and the optimal S/N ratio cannot be easily obtained.